Analyzing the DFIG Inclusion in an Electrical Grid

, October 21-23, 2015, San Francisco, USA Analyzing the DFIG Inclusion in an Electrical Grid Rubén Tapia-Olera, Member, IAENG, Agustina Hernandez-Tolentino, Omar Aguilar-Mejia and Antonio Valderrabano-Gonzalez Abstract With the increasing penetration of wind energy conersion system in distribution grids, utilities are requiring these renewable resources to proide reactie power supports during steady state and transient operating conditions. Wind energy conersion systems with doubly fed induction generators are able to independently control actie and reactie power. This paper examines the reactie power control capability of doubly fed induction generator connecting to distribution grid with and without the inclusion of capacitors banks. The dynamic response characteristics of the wind power generator in the case of fault in the network to assure the secure and reliable operation of wind farm is presented. The stator-flux-oriented ector control principle is applied to build a model of the doubly fed induction generator en dq synchronous coordination system and the PSCAD/EMTDC simulation software is employed to inestigate its performance into distribution grid of eleen nodes. Index Terms DFIG, distribution grid, reactie power. I. INTRODUCTION IND power may be considered as one of the most Wpromising renewable energy sources after its progress during the last three decades. Howeer, its integration into power systems has a number of technical challenges concerning security of supply, in terms of reliability, aailability and power quality [1-3]. Since the penetration of wind power generation is growing, system operators hae an increasing interest in analyzing the impact of wind power on the connected power system. For this reason, grid connection requirements are established. In the last few years, the connection requirements hae incorporated in addition to steady state problems, dynamic requirements, like oltage dip ridethrough capability [4-5]. The most common requirements under these disturbances are low oltage ride through (LVRT) which usually implies: oltage profile immunity, reactie current injection, actie and reactie power limitation under fault and recoery and limitation in reactie Manuscript receied May 29, 2015; reised June 26, 2015. This work was supported by PROMEP: Redes Temáticas de Colaboración under the project titled: Fuentes de Energías Alternas. Rubén Tapia-Olera, Agustina Hernández-Tolentino and Omar Aguilar- Mejia are with the Engineering Department at Polytechnic Uniersity of Tulancingo, Tulancingo, Hidalgo, CO 43629, Mexico (corresponding authors to proide phone: 775-755-8319; fax: 775-755-8321; e-mail: ruben.tapia, [agustina.hernandez], [omar.aguilar]@upt.edu.mx). Antonio Valderrabano-Gonzalez is with Mechatronics Department at Uniersidad Panamericana Campus Guadalajara, Zapopan, Jalisco, CO 45010, Mexico (e-mail: aalder@up.edu.mx). energy consumption [5-6]. Therefore, in electrical grids where plans include wind generation systems a detailed analysis is required, it must be contain releant technical aspects of the operation in steady state and transient conditions. Howeer, the model has an important role to ensure correct results. In the past, when a serious failure in the electrical grid was presented wind energy conersion systems (WECS) were disconnected. Howeer, due its penetration has been increased it is necessary that all time must remain connected, because they proide a large amount of power, therefore, it can cause major problems to the grid [7]. The continuous operation under different perturbations is not easy to get, the control structure must ensure proper dynamic response. The major literature includes control structure to doubly fed induction generator (DFIG) connected to an infinite bus but not contain all structure electrical grid [1, 8-9]. Analyzing a control structure with a suitable model system largely ensure proper operation in actual conditions. Therefore, a comprehensie study of electrical grid including WECS with detail control structure considering the effect of major disturbances and the behaior of oltage and power flows is required. II. DFIG-BASED WIND TURBINES Fig. 1 shows the schematic diagram of DFIG-based wind generators. The DFIG is an induction machine with a wound rotor where the rotor and stator are both connected to electrical sources, hence the term doubly- fed. The rotor has three phase windings that are energized with three-phase currents. These rotor currents establish the rotor magnetic field. The rotor magnitude field interacts with the stator magnetic field to deeloped torque. The magnitude of the torque depends on the strength of the two fields. The system ensures efficient power conersion due to ariable rotor speed, which adjusts itself automatically in accordance with preailing wind speeds. Speed ariability is made possible by the directionally dependent transfer of slip power ia the frequency conerter, which changes as follows [10]: a) In the sub-synchronous operating mode (partial load range), the stator of the DFIG supplies power to the grid and also the slip power to the rotor ia slip rings and the frequency conerter; b) In the super-synchronous operating mode (nominal load range), both the stator output power and the rotor slip power are fed into the grid. The stator is connected to the network by a transformer, while the rotor connection to the network is performed through a bidirectional frequency conerter (formed by two power electronic conerters AC/DC, reersible) and a

, October 21-23, 2015, San Francisco, USA transformer. The grid side conerter always runs at mains frequency, the ector control can independently absorb or inject actie power through the machine rotor and can control the reactie power exchanged between the machine and the electrical grid. The rotor side conerter instead, works at ariable frequency depending on the operating point. This topology is named back-to-back (BTB) conerter. The control structure is the main part to regulate de DFIG performance. Fig. 1. Schematic diagram of DFIG-based wind generators. III. CONTROL SCHEME The BTB conerter is formed by two oltage source conerters (VSC) sharing a DC bus that allow independent control, Fig. 1. To achiee control in power conersion with minimal impact on the conentional grid a BTB conerter switched by pulse wih modulation (PWM) techniques is used. A. Source side conerter control The objectie of the grid-side conerter is to keep the dclink oltage constant regardless of the magnitude and direction of the rotor power. The ector-control method is used as well, with a reference frame oriented along the stator oltage ector position, enabling independent control of the actie and reactie power flowing between the grid and conerter. The conerter is current regulated, with the d-axis current used to regulate the dc-link oltage and q-axis current component to regulate the reactie power. Fig. 2 shows the schematic control structure of the grid-side conerter [11]. The oltage equations in synchronously rotating dq-axis reference frame are [12]: di = ω L i (1) 1 Ri + Lr e r cq + di = ω (2) cq1 cq Ri + L + L i + cq cq r e r The angular position of the grid oltage is calculated as = = tan 1 cβ θe ωe (3) cα where cα y cβ are the conerter grid-side oltage stationary frame components. The d-axis of the reference frame is aligned with the grid oltage angular position θ e. Considering that the oltage grid amplitude is constant, cq is zero and is constant. The actie and reactie power will be proportional to i and i cq respectiely. Assume the grid side transformer connection is start; the conerter actie and reactie power flow are, P = 3( i + i ) = 3 i (4) c cq cq Q = ( i + i ) = 3 i (5) c 3 cq cq cq The control scheme uses a current decoupling for i and i cq, to determine the component i, the oltage difference in DC bus is used. While, the alue for i cq is calculated by the displacement factor in grid side inductor. For the internal control loop it is necessary to design the proportional integral (PI) controller s parameters, which are obtained by application of the Laplace transform to (1) and (2) that represent the grid-side conerter oltages in its dqcomponents. The reference for the oltages alues * and * cq can be obtained by [11]: * ' = + ( ω L i + ) (6) * cq e r cq ' = ( ω L i ) (7) cq e r These reference alues are the inputs used in the PWM technique in order to guarantee the DC oltage leel and the required power factor. Fig. 2. Control structure of the grid side conerter.

, October 21-23, 2015, San Francisco, USA B. Rotor side conerter control The rotor side conerter proides the excitation of the induction machine. So, it is possible to control the torque, therefore, the speed of the induction generator and the power factor of the stator terminals. The rotor side conerter supplies a ariable excitation frequency depending on the conditions of the wind speed. The first step for the rotor side conerter control is to determine the instantaneous stator rotating flux ector location θ s. ψ ψ sα sβ ( ) sα Rsis α ( R i ) = = sβ s sβ, θ = tan s 1 ψ s ( ψ The next step is to generate the rotor current references. For this purpose two PI regulators with the inputs of Q ref and ω r are used, Fig. 3. β sα IV. SIMULATION RESULTS The test utility network configuration used in this paper is shown in Fig. 4 as a single line diagram. It consists in a grid of 11 buses with oltage at 13.8 kv. There are 9 loads in the system with total actie and reactie ratings of 1.1475 MW and 0.7101 MVARs, respectiely. There are two wind farms connected in 7 and 10 buses through back to back conerter with a control structure shown in section III. In order to erify the control scheme of the DFIG inside an electrical grid, we study the demand of actie, reactie power and its capability to remain connected to the distribution grid with security. Two cases are analyzed in the PSCAD software. A. Case 1 This case presents the power flow analysis of electrical grid with wind turbines. Initially the system is inactie; all ariables hae alues equal to zero, after a transient period are stabilized around of a constant alue at steady state. The 9 loads are of 150 kva each one that is the nominal capacity of the transformer with a power factor of 0.85. Table I shows the measurements for different wind speeds from 6 to 15 m/s. The generation units should remain around ) (8) unity power factor at the connection point due to the control scheme. The leel of reactie power is controlled by the conersion system by assigning a reference alue equal to zero. It can be seen that turbines are operating with a factor close to unity. Furthermore, we can see that the DFIG has the ability to regulate the reactie power flow exchanged with the grid in steady state at different wind speeds. The control structure guaranties such behaior. TABLE I MEASUREMENT OF POWER FLOW IN EACH GENERATION UNIT Wind DFIG 1 DFIG 2 Speed P1 Q1 P2 Q2 FP1 m/s MW MVArs MW MVArs FP2 6 0.169-0.017 0.99 0.173 0.003 0.99 8 0.209-0.001 0.99 0.201-0.009 0.99 9 0.276 0.010 0.99 0.287 0.012 0.99 12 0.568 0.0005 0.99 0.570 0.001 0.99 15 1.205-0.079 0.99 1.253-0.054 0.99 It can be seen that system s control has a good response. Howeer, the control system operates with unity power factor, there is no reactie power exchange with the distribution network. Therefore, it is examines the inclusion of a capacitor bank in the common connection point in order to coer the supply of reactie power demanded by the loads, transformers and transmission line to aoid problems of stability and improe oltage profile. The comparison results with and without inclusion of capacitors banks are presented in Table II. It is conenient to analyze how are the adjusted and modified the line losses; the total demand of actie and reactie power is 1.1475 MW and 0.7101 MVARs, respectiely. The total actie power generated without the inclusion of the capacitor bank is 1.173 MW and the reactie power is equal to 0.7785 MVARs. Table II shows that without capacitors banks the actie power demand is supplied by 98.99% by wind turbines, while that 1.01% is coer by the AC grid. For the case of reactie power the results are different, the 100% reactie power demanded by the system is taken from the conentional AC network, hence the importance to include the capacitors banks to compensate reactie demand. Fig. 3. Control structure for rotor side conerter.

, October 21-23, 2015, San Francisco, USA AC# AC/DC/AC# AC/DC/AC# Fig. 4. Single line diagram of test electrical grid. The total actie power generated is ery similar for both cases. On the other hand, the capacitors banks coer 100% of the reactie power demand for both loads and losses. It can be seen that the losses in the network decreases with integration of the capacitor banks from 68.4 KVARs to 57.9 KVARs and 10.2KVARs supplied to AC system. Similarly, the connection of distributed generation systems close of the loads exhibits two main adantages: a) the loads are powered by generating systems from alternatie energy sources rather than conentional sources, b) improing the oltage profile making the system more robust. The oltage profile is better with capacitors banks inclusion all magnitudes are close to 1.0 pu, nodes 8-11 hae similar performance, Table III. The phase angles maintain their alues more close to each other respect the systems without capacitors, Table III. TABLE II COMPARISON RESULTS WITH AND WITHOUT INCLUSION OF CAPACITORS BANKS. Line Without capacitors banks With capacitors banks from to P Q S P Q S MW MVArs MVA MW MVArs MVA 1 2 0.037 0.779 0.779 0.0134-0.0102 0.016 2 3 0.254 0.161 0.300 0.256 0.162 0.302 2 5-0.357 0.516 0.627-0.371-0.254 0.449 3 4 0.127 0.080 0.150 0.128 0.080 0.151 5 6-0.489 0.429 0.650-0.500-0.339 0.604 6 7-0.618 0.344 0.707-0.632-0.423 0.760 G1 7 0.566 0.0002 0.566 0.574 0.377 0.686 7 8-0.183 0.256 0.314-0.190-0.134 0.232 8 9-0.310 0.177 0.356-0.319-0.217 0.385 9 10-0.438 0.094 0.447-0.450-0.302 0.541 G2 10 0.570-0.0007 0.570 0.583 0.391 0.701 10 11 0.126 0.079 0.148 0.128 0.081 0.151 2 1-0.016-0.753 0.753-0.013 0.0107 0.017 3 2-0.254-0.160 0.300-0.255-0.161 0.301 5 2 0.370-0.504 0.625 0.373 0.258 0.453 4 3-0.127-0.080 0.150-0.127-0.080 0.150 6 5 0.500-0.419 0.652 0.503 0.342 0.608 7 6 0.631-0.333 0.713 0.631 0.425 0.760 8 7 0.185-0.248 0.309 0.191 0.135 0.233 9 8 0.314-0.165 0.354 0.322 0.221 0.390 10 9 0.443-0.080 0.450 0.454 0.31 0.549 11 10-0.126-0.079 0.148-0.128-0.08 0.150 12 m/s, the reactie power control is regulate by side source conerter. When the oltage exceeds a critical limits turbine the reactie power controller supplied or absorbed to preent oer oltage or under oltage, which could result in the disconnection. TABLE III MAGNITUDE AND PHASE ANGLE OF NODAL VOLTAGES. Without capacitors With capacitors banks banks Node Voltage Phase Voltage Phase (pu) (degrees) (pu) (degrees) 1 1.0 0.0 1.0 0.0 2 0.97 0.9799 0.99-0.08 3 0.97 0.892 0.99-0.168 4 0.97 0.87 0.99-0.190 5 0.97 1.71 1.0 0.107 6 0.97 2.18 1.01 0.266 7 0.97 2.81 1.02 0.507 The system is subjected to a three-phase short-circuit fault at bus 9 after 10 ms is deliered. At t = 0.2 s the oltage at terminals G1 has decreased about 50% of its nominal alue, while the actie and reactie powers present a transient period and reach steady state alues, Fig. 5. In the case of the generator 2, Fig. 6, the reduction in oltage has a leel of approximately 65% of the nominal alue; the powers after 0.08 milliseconds return to its steady-state alue. The results exhibit good control structure performance when a DFIG is included in a typical electrical grid. B. Case 2 The second case analyzes the behaior of DFIG in face to oltage drop across the network considering a wind speed of

, October 21-23, 2015, San Francisco, USA Fig. 5. Generator 1 when there is a short circuit fault 10 ms at t = 0.2 s: a) terminal oltage; b) actie and reactie power. Fig. 7. Three phase oltage at connection node to the grid when a shortcircuit fault is applying at t = 0.2 s: a) G1 unit; b) G2 unit. Fig. 6. Generator 2 when there is a short circuit fault 10 ms at t = 0.2 s: a) terminal oltage; b) actie and reactie power. The impact of the oltage drop in each generation unit depends on the proximity where the fault occurs in the distribution network. For this study the fault is located closer to the generator 2. Fig. 7 presents the oltage behaior in the three phases at connection point of each wind turbine. During fault time the oltage magnitude of terminals G2 is less than G1, at t = 0.210 s the fault is cleared. The units tend to stabilize, and at t=0.220s attain its steady state alue, showing a typical waeform at steady state condition for power conerters. Control structure adequately fulfilling its task with transient response when hard perturbations in an electrical grid are presented, the results are in accordance with preious case. The impact of the oltage drop in the grid stability depends on the duration and seerity. Fig s. 8 y 9 show when a fault with a duration of 87 ms is applied, the terminal oltage of both units has decreased more than 50% of their nominal alue, as a result, the transient period in the actie and reactie power of connection point is more prolonged, but the system reach its steady state condition about t = 0.35 s. Fig. 8. Generator 1 when a short circuit fault is applying at t= 0.2 s: a) terminal oltage; b) actie and reactie power. The analysis of two doubly fed induction generators in distribution network has been presented in this paper, each unit represents a wind farm with a detailed control structure. For the analysis and ealuation of the control strategy, a study of power flows and dynamic response were deeloped. It is concluded that it is necessary to include a capacitor bank in the common point of connection to the network, because the system controller of DFIG operates to a unity power factor. Furthermore, with the inclusion of the capacitor bank is compensated demand for reactie and the

, October 21-23, 2015, San Francisco, USA oltage profile is improed. The losses present in case 1 are smaller with inclusion of the capacitor bank but the main contribution is obsered in balancing the flows of the system under study. Another important aspect is the stability analysis of DFIG connected to network in face to oltage dips. REFERENCES [1] Mostafa Soliman, O. P. Malik, Daid T. Westwick, Multiple Model Predictie Control for Wind Turbines With Doubly Fed Induction Generators, IEEE Trans. Sustainable Energy, ol. 2, No. 3, pp. 215-225, 2011. [2] Hoa M. Nguyen, D. Subbaram Naidu, Adanced Control Strategies for Wind Energy Systems: An Oeriew, in Proc. 2011 IEEE Power Engineering Society Power Systems Conf., pp. 1-8. [3] Zamani, M.H., Riahy, G.H., Foroushani, R.Z., Introduction of a new index for ealuating the effect of wind dynamics on the power of ariable speed wind turbines, in Proc. 2008 IEEE Power Engineering Society Transmission and Distribution Conf., pp. 1-6. [4] Ghofrani, M., Arabali, A., Etezadi-Amoli, M., Baghzouz, Y., Operating resere requirements in a power system with dispersed wind generation, in Proc. 2012 Power Engineering Society Innoatie Smart Grid Technologies, pp. 1-8. [5] Rahmann, C., Haubrich, H.-J., Moser, A., Palma-Behnke, R., Vargas, L., Salles, M.B.C. Justified Fault-Ride-Through Requirements for Wind Turbines in Power Systems, IEEE Trans. Power Systems, ol. 26, No. 3, pp. 1555-1563, 2011. [6] Hua Geng, Cong Liu, Geng Yang, LVRT Capability of DFIG-Based WECS Under Asymmetrical Grid Fault Condition, IEEE Trans. Industrial Electronics, ol. 60, No. 6, pp. 2495-2509, 2013. [7] K. Vinothkumar, M.P. Selan, Noel scheme for enhancement of fault ride-through capability of doubly fed induction generator based wind farms, Energy Conersion and Management, ol. 52, No. 7, pp. 2651-2658, July 2011. [8] Van-Tung Phan, Hong-Hee Lee, Performance Enhancement of Stand-Alone DFIG Systems With Control of Rotor and Load Side Conerters Using Resonant Controllers, IEEE Trans. Industry Applications, ol. 48, No. 1, pp. 199-210, 2012. [9] J. Hu, H. Nian, B. Hu, Y. He, and Z. Q. Zhu, Direct actie and reactie power regulation of DFIG using sliding-mode control approach, IEEE Trans. Energy Conersion, ol. 25, No. 4, pp. 1028 1039, 2010. [10] Gonzalo Abad, Jesús López, Miguel Rodríguez, Luis Marroyo, Grzegorz Iwanski, Doubly Fed Induction Machine, IEEE Press, cap.9, 2011. [11] R. Pena, J.C.J.C Clare, G.M. Asher, Doubly fed induction generator using back-to-back PWM conerters and its application to ariablespeed wind-energy generation, IEE Proceedings Electric Power Applications, ol. 143, No. 3, pp. 231-241, 1996. [12] Jatin Nath Wani and Artie W. Ng., Paths to sustainable energy, Intechopen, cap. 14, 2010. Fig. 9. Actie and reactie power of generator 2 when a short circuit fault is applying at t= 0.2 s with a duration of 87ms. V. CONCLUSION The results show that the controller has a fast response and allows wind turbines successfully get oer after the fault period and return to their steady state condition. Analysis of detailed control structure when including wind systems in the electrical grid with specialized software presents actual characteristics, which can be used for the planning and operation of electrical networks.